Rheem Prestige 9.9GMP with Intermittent DHW

We built a new house with a Rheem Prestige 9.9 GMP Combi-Boiler. The model is RCBH199DVLP. We are having trouble with intermittent domestic hot water.
The DHW stopped entirely two weeks ago. It would flow, but would not heat. The DHW side never fired. The heat side continued to fire with no problems.
This was diagnosed (I think correctly) as a faulty three-way valve. The way these are set up (as I understand it) is the DHW will take preference when the unit detects flow and shut down the heat side.
Since the valve was been replaced, DHW is returned. It works fine sometimes, but sometimes will only run cold. We are going to get the techs back out Monday, but I was thinking on this problem and thought I might have an idea.
Is it possible when they were working with the unit they changed the setting that makes the DHW higher priority? I am speculating that the DHW is not firing while the heat is firing. Is that a possibility with these units?
Any help is appreciated. Our new homeowners are very gracious, but they bought a new house and would like it to work. Thank you!
Comments
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do you have pictures?
is there any zone controllers? are they set correct?
zone valves or circs? let's see pictures, wide angle, showing everything, floor to ceiling
known to beat dead horses0 -
it is triggered by a flow switch, maybe .35 to .50 gpm. Somewhere near the switch or in the unit is a small strainer, look in your manual for location. As they plug it prevents the unit from getting adequate flow to trigger. It is an easy thing to check and it may need periodic service.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
Post photos of DHW piping into and out of combi, and recirculation pipes.
Any pattern to when it runs cold?
Any error codes?
What is temperature of the incoming cold?
The boiler heating component is working well?
I second to make sure filter is clean.
This symptom is sometimes caused by crossover between hot and cold pipes somewhere else in the house. Shower valves or unchecked recirculation lines are some potential sources.
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Sorry about the slow reply. Helping to cater a wedding. Here are the boiler pictures:
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No error codes. I have not discerned a pattern. Incoming temp is approx 54F year round. Boiler heat side is fine, no problems. I will check the filter. We piped the whole house so I am pretty sure there is no crossover.
Thank you!
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Did you get this sorted out?
I also suspect it may be related to flow not always meeting the minimum requirements.
Are there any mixing valves anywhere on DHW side?
What's the flow rating on the bathroom faucets or showers?
What temperature is the DHW set to? Heating side? have you tried other temps?
I would still test for crossover by closing the isolation valve at the tankless and opening a hot faucet upstairs, make sure the flow comes to a complete stop.
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